Layer-Structured Zintl Phases as Novel Thermoelectric Materials
Shuai, Jing 1988-
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Thermoelectric materials have been fascinated extensive interest in the last two decades due to the potential applications in waste-heat recovery from industrial processes, automobiles, and renewable energy sources. In pursuit of potential thermoelectric candidates, Zintl phases have recently gained interest because of their characteristics desired for use in thermoelectric devices for power generation: complex structures, extensive opportunities to tune transport properties, narrow band gaps, and thermal and chemical stability. In this huge pool of Zintl compounds, the two-dimensional layered CaAl2Si2-typed Zintls, especially antimony-based Zintl compounds, have been demonstrated to be promising TE materials for middle- to high-temperature applications. Here, we report the rarely studied bismuth-based Zintl phases (Ca,Yb,Eu)Mg2Bi2 with competitive thermoelectric performance. This thesis first describes the thermoelectric characterization and optimization of these phases, including eliminating the Bi impurity, Na substitution to boost power factor, as well as as understanding the band engineering and strain field fluctuation in solid solutions. In addition, since the fabrication method for synthesizing the Zintl compounds here is different from the most reported melting method, the comparison between these two methods based on the classical Sb-based Ca1-xYbxZn2Sb2 compounds will be included, which allows us to further understand the defect-controlled p-type Zintl phases. For application, thermoelectric generators should consist of both n- and p- type legs with equivalent performance. However, through several decades’ efforts, it still remains very difficult to make competitive n-type Zintl materials until the discovery of n-type Mg3+xSb2-based Zintl phase. Given the reported low electrical conductivity and low average ZT, it motivates us to further improve it through Nb partial substitution, resulting in enhanced average ZT value to about 1 across the entire measured temperature. Moreover, doping with the ideal hole dopants (Na+) on its corresponding p-type Mg3Sb2 Zintls has also been investigated, allowing for effectively increasing the carrier concentration and optimization of the thermoelectric performance. Besides the Zintl phases, the final part introduces another excellent p-type thermoelectric materials MgAgSb with potential power-generation application near room temperature. The works highlight the discovery of complex bulk thermoelectric materials with intrinsically low lattice thermal conductivity, which would stimulate many of the recent advances in thermoelectrics.